The present invention is applicable to substantially any process for drilling the borehole of a well with an oil-based drilling fluid in a location in which the wellbore may encounter hydrogen sulfide. Treatment of drilling fluid or mud is very important when drilling in areas where hydrogen sulfide (H.sub.2 S) may be encountered. H.sub.2 S is a highly toxic and corrosive acidic gas having, at low concentrations, the distinctive odor of rotten eggs. When H.sub.2 S enters the drilling fluid, it reacts with the alkaline drilling fluid and is converted into bisulfides (HS.sup.-) and sulfides (S.sup.-2).
The term "sulfides" as used here includes all water-soluble species, H.sub.2 S, HS.sup.-, and S.sup.-2, which co-exist in a sulfide-water system. The relative proportion of each species at equilibrium depends upon pH. For solutions with a pH of 7 to 13, typical for drilling fluids, bisulfides are the predominate species. If the pH drops below 7, H.sub.2 S predominates, and above pH 13, sulfides predominate. Treatment of drilling mud with caustic soda and lime is often practiced where the presence of H.sub.2 S is suspected. When soluble sulfide species accumulate in drilling mud, even small decreases in pH can generate large volumes of H.sub.2 S gas in the drilling mud. To avoid reconversion of sulfides into H.sub.2 S, it is common practice to react the sulfides into a more chemically inert form, such as precipitation as an insoluble metal sulfide.
The term "sulfide scavenger" refers to any drilling fluid additive that can react with one or more sulfide species and can convert them to a more inert form. Scavenger compounds are added to the drilling fluid at levels sufficient to provide a slight excess of scavenging compound over the amount of H.sub.2 S present or anticipated in the drilling fluid in order to ensure quick and complete removal. Zinc-based additives, such as zinc carbonate, zinc hydroxide and organic zinc compounds, provide effective scavenging by a rapid and irreversible reaction with sulfides to form solid zinc sulfide. The most prevalent commercial zinc-based scavengers are Mil-gard (Milchem), Coat 45 (Baroid) and Sulf-X(IMCO).
Zinc is often the preferred metal ion to react with sulfide because of its compatibility with drilling fluid and its effectiveness in precipitating sulfide ions. U.S. Pat. No. 3,928,211 describes a process for scavenging H.sub.2 S from aqueous drilling fluids and describes several known processes for removing H.sub.2 S from drilling fluids. A class at zinc carbonate, basic zinc carbonate, and zinc hydroxide are listed as effective for scavenging sulfide. U.S. Pat. 4,252,655 covers a process for removing sulfide ions from both water and oil-based drilling fluids with organic zinc chelates.
If the hydrogen sulfide can be chemically bound while in the borehole, then it will not reach the surface in a potentially hazardous form. If the reaction is fast enough, and pretreatment concentrations of scavenger are adequate, then pipe failure due to corrosion can also be avoided. Limiting the amount of scavenger added to that required for effective control of H.sub.2 S entering the mud reduces operating costs and lessens the change of impairing mud rheology by overtreatment.
Previous field testing methods for determining sulfide scavenger requirements in drilling operations have focused upon sulfide analysis. U.S. Pat. No. 3,928,211 mentions that the amount of zinc required for scavenging may be determined by analyzing the mud for sulfide. U.S. Pat. No. 4,252,655 suggests the need to monitor drilling mud for sulfide ions to determine when additional zinc chelate must be added. A device called the "Mud Duck" described in "H.sub.2 S Detector Aids Drilling Safety", by S. H. Calmer in Oil and Gas Journal, Nov. 19, 1979, page 160, continuously measures the total soluble sulfides in any aqueous mud system using ion-selective electrodes, and relates these measurements to the dissolved H.sub.2 S gas in equilibrium with the mud.
A state of the art paper entitled "Chemical Scavengers for Sulfides in Water Based Drilling Fluids" by R. L. Garrett, R. K. Clark, L. L. Carney and C. K. Grantham, Sr. in the Journal of Petroleum Technology, June, 1979, page 787, discusses the chemistry of commercial scavengers, the parameters that affect the reliability of such materials and the problems affecting scavenger use. The Garrett Gas Train method for sulfide analysis is described as a simple, yet accurate field procedure for monitoring sulfides in drilling fluid filtrate. The same article describes a method developed by a service company that uses titration to analyze for total alkaline-soluble zinc, but not the reacted zinc product, zinc sulfide. The authors also identified a need for realistic rig-site monitoring test to measure the amount of available scavenger in drilling fluid.
Although there are many analytical techniques available for the quantitative determination of zinc, few are capable of differentiating forms of the metal. Speciation capability is critical for isolating unspent scavenger from spent scavenger (zinc which has reacted with sulfide to form zinc sulfide). These analytical methods which can offer speciation capability (X-ray diffraction, photoelectron spectroscopy) are inappropriate for field applications due to cost, lack of ruggedness, size, or extensive utility requirements. One solution to this dilemma is to invoke a chemical separation prior to an analysis for total zinc by a field-worthy method.